A major problem in the regulation of metabolism is to understand those factors relevant to the determination of intracellular enzyme concentrations in eukaryotic cells. In contrast to extensive knowledge concerning protein synthesis, relatively little is known about controls of protein turnover. One hypothesis assigns a major role to apoprotein - pyridoxal phosphate interaction in the modulation of intracellular turnover of pyridoxal phosphate requiring enzymes. The role of cofactor binding will be assessed by studying turnover, in vivo, in pyridoxine deficient rats; two representative enzymes, of the sulfur amino acid pathway, will be used for study. Further in vitro studies are designed to more precisely define the parameters relevant to the turnover of these representative pyridoxal phosphate enzymes. Experimental goals will include studies of inactivation system itself, and early products of the degradation reactions. The results will have relevance to the understanding of the control of intracellular enzyme levels in eukaryotic cells, as well as to a clarification of mechanisms obtaining in inborn errors of metabolism secondary to deficiency of pyridoxal phosphate enzymes. The work will also have relevance to recent experimental and conceptual advances in studies of the aging process.